Refrigerating apparatuses including refrigerant circuits performing refrigeration cycles by circulating refrigerant have been known conventionally, and are being used widely for indoor air conditioning and refrigerator cooling, for example. Some of the refrigerating apparatuses of this type, expanders are provided in the refrigerant circuits for power recovery in place of expansion valves.
Patent Document 1 discloses a refrigerating apparatus including such an expander. The refrigerating apparatus includes a compressor, a radiator, an expander, and an evaporator which are connected sequentially. Carbon dioxide is filled as refrigerant in a refrigerant circuit. In the refrigerant circuit, polyalkylene glycol is used as refrigerating machine oil for lubricating respective sliding portions of the compressor and the expander. The compressor and the expander are mechanically coupled to each other through a rotary shaft.
During cooling operation of this refrigerating apparatus, the refrigerant discharged from the compressor flows into the expander after dissipating heat in the radiator. In the expander, the expansion power when the refrigerant is expanded is recovered as rotational force of the rotary shaft. The refrigerant in a gas/liquid two-phase state flowing out from the expander flows into an oil separator. Here, the two-phase gas/liquid refrigerant contains oil utilized for lubricating the expander. Therefore, in the oil separator, the oil is separated from the two-phase gas/liquid refrigerant, and is retained in the bottom of the oil separator. The refrigerant from which the oil is separated in the oil separator flows into the evaporator. In the evaporator, the refrigerant absorbs heat from indoor air to cool the indoor air. The refrigerant evaporated in the evaporator is sucked into the compressor to be compressed again.
While, an oil return pipe communicating with the suction side of the compressor is connected to the bottom of the oil separator in Patent Document 1. Accordingly, the oil separated in the oil separator as described above is sucked into the compressor through the oil return pipe to be utilized for lubricating the sliding portions of the compressor. Thus, in this refrigerating apparatus, the oil is separated from the refrigerant on the outflow side of the expander and is sent to the suction side of the compressor. Therefore, this refrigerant apparatus can prevent the oil flowing out from the expander from flowing into the evaporator. Consequently, degradation of the heat transfer performance of the evaporator, which is caused by adhesion of the oil to the heat transfer tubes of the evaporator, can be prevented, thereby ensuring cooling performance of the evaporator.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-139420